linux-kernel - Re: some thoughts about TSC based delay

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Message-ID: <20071121200958.GA30663@elte.hu>
Date:	Wed, 21 Nov 2007 21:09:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Marin Mitov <mitov@...p.bas.bg>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: some thoughts about TSC based delay_tsc()


* Marin Mitov <mitov@...p.bas.bg> wrote:

> On Wednesday 21 November 2007 09:27:54 pm you wrote:
> > * Marin Mitov <mitov@...p.bas.bg> wrote:
> > > Hi Ingo,
> > >
> > > The patch is quite good ;-) but we forget when it is needed :-( In 
> > > fact we need it only for PREEMPT SMP kernels - it could hurt 
> > > PREEMPT UP kernels (no migration possible), so no need for 
> > > preempt_disable()/preempt_enable().
> > >
> > > In short the old version of delay_tsc() is good for UP kernels and 
> > > NON PREEMPT SMP kernels too.
> >
> > please reply to the public list, so that discussions do not get lost.
> >
> > i dont think there's any problem: udelay() is about _wasting_ cycles -
> > it's what drivers use for short delays.
> 
> Sure for the thread executing udelay(), but not for the other ready 
> threads which should also wait till preempt_enable() to grab the same 
> cpu even for PREEMPT (UP or SMP) kernels (or I misunderstand 
> something?).

on non-PREEMPT kernels there's no real difference between old and and 
new code because the kernel is not preemptible. So we can use the new 
code unconditionally.

	Ingo
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